Recently, a portable terminal has become a necessity in a modem life, enjoyed by everyone regardless of age or gender. Accordingly, service providers and portable terminal manufacturers competitively develop products (or services) that can differentiate themselves from others.
For example, the portable terminal has developed into a multimedia device in which a phonebook, game, SMS, e-mail, wake-up call, MP3, calendar, digital camera, multimedia messaging and wireless internet service are available, to provide a variety of services.
In order to ensure mobility of the portable terminal, the portable terminal provides a handoff technology, which maintains a call connection by connecting a call to an adjacent base station instead of disconnecting the call when the portable terminal moves outside a service area of a serving base station currently engaged in a communication.
In addition, the portable terminal provides a voice service through a circuit switched network including a public switched telephone network (PSTN) and provides a packet service through an internet protocol (IP) network including Internet or packet switched data network (PSDN). Also, the portable terminal provides a VoIP function to provide the voice service through IP network. Due to advancement of the IP network such as Internet and overcoming a limitation of a voice bandwidth of 56 kbps in the circuit switched network, the VoIP function can enable a high quality voice call as well as an international call with a low cost internet fee. The VoIP functionality can also be provided through a wireless local area network (WLAN).
Accordingly, when a wireless LAN network capable of performing the VoIP function is available, a user of the portable terminal may want to use the voice service over the wireless LAN network instead of the mobile communication network; however, in order to use the VoIP function during when a call connection is being performed, the call connection that is already established needs to be disconnected to perform the VoIP function.
In other words, the portable terminal supports only for a handoff within the mobile communication network or within the wireless LAN network but does not support the handoff between the mobile communication network and the wireless LAN network so that the call connection that is already established needs to be terminated.
Accordingly, in order to solve the above problem, there exists a need for an apparatus and a method for performing a handoff between the mobile communication network and the wireless LAN network without requiring disconnecting a call.